1. Field of the Invention
The present invention is related to height control circuits of the type employed in video displays which employ raster scan CRT elements. More particularly the present invention relates to an improved height control circuit for maintaining constant height output when receiving dynamically changing frequencies at the input.
2. Description of the Prior Art
Heretofore the prior art vertical height control circuits did not provide means for maintaining a constant height output while receiving a dynamically changing variable frequency input. Prior art vertical height control circuits have been provided with multiband adjustments which permit the height control circuit to be operated within a narrow band of frequencies. However, such multi-frequency vertical height control circuits require a separate voltage reference input for each of the frequency bands. Most recently monitors have been provided with means which sense the polarity of the incoming signal and/or the frequency of the incoming signal to select one of a plurality of frequency reference voltages. The problem that exists with these frequency band selecting circuits is that the vertical height output signal from the ramp generator varies in height within the band selected.
It would be extremely desirable to provide a simplified, extremely accurate and inexpensive vertical height control circuit which maintains an accurate constant vertical height while receiving a wide range of variable frequency input signals.